There is a continuing and increasing understanding of the manner in which the growth of a cell in vivo is regulated. Much attention has been directed toward the isolation of growth regulatory polypetides from activated leukocytes. Of particular interest are those polypeptides which selectively inhibit the growth of neoplastic cells while not inhibiting the growth of surrounding normal tissue. Interferons (IFN) have shown growth inhibitory activity against a variety of tumor cells. There has also been reported synergy between low levels of different types of IFN or of IFN and other growth inhibiting peptides, such as lymphocyte-derived tumor necrosis factor-.beta.(TNF-.beta.). TNF-.beta. as well as TNF-.alpha., derived from myelocytic cells, displayed cytostatic and/or cytocidal activity against several transformed cell lines, but normal cells were unaffected. TGF-.beta. is produced and released by a variety of cells, including T-cells and monocytes, and can both stimulate and inhibit cell proliferation, depending largely on the cell type. TNFs and TGF-.beta.s are produced by both activated macrophages and T-lymphocytes, and are structurally and functionally conserved among different species and cell types of the immune system.
Other compounds have also been reported as having selective activity against neoplastic cells. Since there is substantial interest in being able to control tumor cell growth, the ability to regulate tumor cell growth is of great interest. However, the naturally occurring compounds usually have a broad range of activities toward different cells and at concentrations which may be effective in inhibiting the growth of neoplastic cells, frequently demonstrate undesired effects toward normal cells. There is, therefore, substantial interest in being able to develop compositions which may be employed at low concentrations when administered therapeutically, while still providing the desired growth inhibitory effects, and with minimal deleterious effects on the normal cells.